Now the other common geometry of astronomy is that of the sphere. So we have two very common geometries in astronomy, we have the disc shape and we have the sphere shape.
Now why would you think astronomy has the predominant shape of a sphere or a disc? Old physics of old physicists would tell you because Newtonian gravity and General Relativity are inverse square laws.
Now that answer is alright for a sphere shape such as the planet Earth or the Sun, but that answer fails for the other predominant shape of a disc shape such as the disc of spiral galaxies or the ecliptic disc or the disc of the Saturn Rings.
So science needs a proper answer to why the two predominant shapes of astronomy are disc shape and sphere shape. An answer that includes both shapes.
And the answer is found in that gravity is EM-gravity of the Maxwell Equations. Gravity is electricity and magnetism. Electricity and magnetism is a force that is 10^40 stronger than Newtonian gravity and being so strong of a force, EM-gravity is Solid-Body-Rotation.
So, that, a particle at the center of Earth, and a particle at the surface of Earth are moving at differential speeds so that they move in rigid-body-rotation, or solid-body-rotation.
Now the planets appear to not be solid-body-rotation, but appearances are often deceptive. For consider that the Sun is moving at 220km/sec while Earth is moving at 29km/sec and there is no way that Newtonian gravity or General Relativity can keep the Earth in orbit around the Sun with those speeds. In ten years time the Earth would be in the Kuiper belt since the Sun is moving almost 10 times faster in Space.
So here is where EM-gravity steps in and saves the planets locked to the Sun. In EM-gravity there is a axis of rotation of the Sun and when extended out in Space to the Oort Cloud it forms a huge sphere and this gravity-cell spins on that axis. Space around the Sun is spinning and the spin makes null the 220km/sec. The spin makes the planets orbit in solid-body-rotation once the spin component is added to the orbital speeds of the planets.
So, to answer the original question of why the two most common geometries in astronomy are the disc shape and the sphere shape is because they are a end result of Solid Body Rotation. The sphere is solid body rotation and the disc shape is orbital speed plus the added factor of Space spin by the gravity-cell.
Google's (and Bing's) searches and archives are top-heavy in hate-spew generated by search-engine-bombing. Google has never made searches fair and clean from bombing practices placed on innocent people. The way to make searches fair is to have transparent true names behind all authors of both Web or Usenet posts. When people can hide behind fake names or omission of true identity, you end up with massive hate spew. And the Google author-archive stopped functioning properly by about May 2012. ?And recently Niuz.biz (Docendi.org) threatens to harm your computer if opening a post of mine.
The solution to the sci. newsgroups is to have the sciences hosted by colleges and universities such as Drexel University hosting sci.math, not by corporations like Google and Microsoft out to make money. And every post should have a true legal name. And people should all be limited to no more than 5 posts per 24 hours, to stop the obnoxious practice of bumping off other posters from the first page of the bulletin board.
Science belongs in education, not in money motivated corporations. Do I hear a?University doing sci.physics, sci.chem, sci.biology, sci.geology, etc ?
Only Drexel's Math Forum has done a excellent, simple and fair archiving of AP author posts for the past several years as seen here: